The invention relates to semiconductor devices fabricated from semiconductor materials.
Silicon, an abundantly available element in nature, has become one of the most commonly used materials for fabricating electrical and electro-optical devices. In its naturally occurring form, silicon is primarily composed of three isotopes of silicon, namely, 92.2% .sup.28 Si 4.7% .sup.29 Si, and 3.1% .sup.30 Si which is also roughly the composition of the silicon crystals used by the silicon device industry. Due to its many desirable properties, in comparison to other semiconductors, not the least of which is the ability to easily grow SiO.sub.2 insulation layers on it, silicon has provided the foundation upon which a semiconductor industry has been built.
Devices fabricated on single-crystal silicon have performance characteristics that are governed by the electrical and physical properties of the silicon itself. Some of the important properties of the single-crystal silicon which affect device characteristics are carrier mobilities, energy band gap, effective mass of the carriers, and thermal conductivity. Carrier mobilities, for example, govern signal transit times and thus place a limit on device speed. Thermal conductivity, on the other hand, governs power dissipation which, in turn, places an upper limit on the packing densities achievable for devices on a chip or the amount of power that can safely be generated in the circuit without significantly degrading circuit performance.
As engineers push the performance of the silicon-based devices, the electrical and physical properties of the silicon often place serious limits on what is ultimately achievable from such devices. For example, as signals and data communications advanced into the higher reaches of the frequency domain, the speed limitations inherent in silicon-based devices caused engineers to shift from silicon to other semiconductor materials, such as GaAs, which exhibit substantially higher carrier mobilities and thus are capable of performance at higher frequencies. The electron mobility in GaAs is over five times greater than that the carrier mobilities typically associated with silicon. This shift to GaAs was felt necessary in spite of the greater technological difficulties associated with fabricating devices from GaAs as compared to silicon. For example, it is appreciably more difficult to grow single-crystal GaAs and to form useful insulating layers on the GaAs.